The open excavation method has always been adopted for subway station construction, wherein the diaphragm wall serves as a supporting structure in the foundation pit, following which the diaphragm wall continuously operates with the subway structure. In previous studies, the diaphragm wall was assumed to have a positive impact on earthquake resistance, and it was usually neglected in the seismic analysis of subway structures. However, the influence mechanism of the diaphragm wall on the seismic performance of subway structures must be further studied. Based on the finite element method of near- field wave motions and the viscous- spring artificial boundary condition, the dynamic responses of a typical two- story and three- span subway station structure with and without the diaphragm wall were compared and analyzed. Furthermore, the influence characteristics and mechanisms of the diaphragm wall on the seismic responses of subway station structures under different site conditions were investigated. Numerical results indicated that the diaphragm wall, which can reduce the overall inter- story displacement of a station structure, is beneficial to the seismic resistance of the side wall and the middle column in the bottom layer. However, the diaphragm wall also magnifies the bending moment and normal stress at the connection between the roof and floor and the side wall. The influence of the diaphragm wall on the internal force and normal stress of the middle column end in the top layer and the middle- span plate end of the structure is related to the site condition. In detail, it has a decreasing effect under hard and medium hard site conditions and a slightly increasing effect under soft site conditions. A possible reason for the response mechanism is that the diaphragm wall increases the lateral stiffness of the structure, thereby reducing the lateral deformation of the station structure. However, the diaphragm wall restricts the side wall deformation, thus increasing the bending moment and internal force at the connection between the roof and floor and the side wall.